In recent years, there have been made researches and developments of various devices using pyroelectric and piezoelectric properties of a ferroelectric. Examples of these devices which utilize the properties of the ferroelectric include ultrasonic sensors and infrared sensors. As the materials of the ferroelectric, perovskite type ferroelectric materials, in particular, PbZr1-xTixO3 (PZT) and (Pb, La)TiO3 (PLT) are excellent in the piezoelectric and pyroelectric properties, and are expected to be used as the materials of the various sensors.
Where the ferroelectric is used for the various sensors, it is required to integrate (unite) the ferroelectric and a signal processing circuit, for reducing the size and noise of the sensor system. For the integration of the ferroelectric and signal processing circuit, it is required to form a thin film of a ferroelectric material and the signal processing circuit on a semiconductor substrate such as a silicon substrate.
Where the thin film of the ferroelectric material is formed on the silicon substrate, it is known to form a buffer layer on the silicon substrate. The buffer layer formed on the silicon substrate makes it possible to reduce a risk of lattice mismatch between the silicon material and the ferroelectric material. The buffer layer also functions as a barrier layer for preventing diffusion of Pb of the ferroelectric material into the silicon substrate, and as an insulating film for electric insulation between a thin film of a lower electrode formed immediately under the thin film of the ferroelectric material, and the silicon substrate.
Based on the finding described above, the inventors of the present invention fabricated an element, by forming the buffer layer in the form of a monocrystalline film of γ-Al2O3 on the silicon substrate, forming the lower electrode in the form of a Pt film on the buffer layer, and forming the thin film of the ferroelectric material on the Pt film, as described in Patent Document 1 identified below. The platinum (Pt) film used as the lower electrode is conventionally used as the electrode material for the thin film of the ferroelectric material such as PZT. Pt has a most dense filler structure, namely, a face-centered cubic lattice (FCC) structure, which has a strong tendency of self-orientation toward the (111) face even on an amorphous material such as SiO2, so that the thin film of the ferroelectric material on the Pt film also has a strong tendency of orientation. However, the Pt film having the strong tendency of self-orientation suffers from a growth of a columnar crystal, and has a problem of easy diffusion of Pb into the substrate along a grain boundary of the columnar crystal. Where the Pt film is used as the lower electrode, the Pt film also has a problem of peeling at the boundary between Pt of the lower electrode and oxides such as SiO2, a monocrystalline film of γ-Al2O3 and PZT. In view of these problems of Pt used for the lower electrode, it has been proposed to replace Pt by an electrically conductive oxide as the lower electrode material, as described in Patent Documents 2-6 identified below.    Patent Document 1: JP-2004-281742 A    Patent Document 2: JP-11-274433 A    Patent Document 3: JP-2000-509200 A    Patent Document 4: JP-8-340087 A    Patent Document 5: JP-8-335672 A    Patent Document 6: JP-8-330540 A
Patent Document 2 discloses an element including a silicon substrate, an impurity diffusion layer and a lower barrier metal layer ((Ti, Al)N) formed on the silicon substrate, a lower electrode thin film of SrRuO3 formed on the impurity diffusion layer and lower barrier metal layer, and a thin film of the ferroelectric material in the form of a thin film of BSTO formed on the lower electrode thin film. Patent Document 3 discloses an element including a silicon substrate, films of TiN and Pt formed on an insulating layer on the silicon substrate, and an electrically conductive oxide layer in the form of a perovskite layer such as lanthanum strontium cobalt oxide (LSCO).
Patent Documents 3, 4 and 5 disclose elements each including a silicon substrate, a buffer layer of MgAl2O4 (magnesium.aluminum.spinel) formed on the silicon substrate, and an electrode in the form of a thin film of SrRuO formed on the buffer layer.